Lithium secondary batteries with positive eletrode compositions and their methods of manufacturing

ABSTRACT

Positive electrodes for secondary batteries formed with a plurality of substantially aligned flakes within a coating. The flakes can be formed from metal oxide materials and have a number average longest dimension of greater than 60 μm. A variety of metal oxide or metal phosphate materials may be selected such as a group consisting of LiCoO2, LiMn2O4, Li(M1x1M2x2Co1-x1-x2)O2 where M1 and M2 are selected from among Li, Ni, Mn, Cr, Ti, Mg, or Al, 0≤x1≤0.5 and 0≤x2≤0.5, or alternatively, LiM1(1-x)MnxO2 where 0&lt;x&lt;0.8 and M1 represents one or more metal elements. Methods for making positive electrode materials are also provided involving the formation of structures with a desired longest dimension, preferably polycrystalline flakes. A cathode coating containing polycrystalline flakes may be deposited onto a conductive substrate and pressed to a desired final electrode thickness.

CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.14/817,135, filed Aug. 3, 2015, which is a division of U.S. patentapplication Ser. No. 12/264,217, filed Nov. 3, 2008, now U.S. Pat. No.9,099,738, issued on Aug. 4, 2015, each of which is entirelyincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to rechargeable lithium secondary batteries thatmay exhibit high power and high energy density. More particularly, theinvention relates to positive electrode compositions and methods ofmanufacturing electrodes for use in lithium secondary batteries.

BACKGROUND OF THE INVENTION

Rechargeable lithium batteries have found an increasing number ofapplications in recent years. The possibility to reduce the size ofthese devices makes them particularly attractive for variousapplications especially for portable electronic devices. Additionallythere are further uses envisioned in the future, particularly inemerging high power applications like portable mechanical tools andhybrid or all-electric vehicles.

The performance of rechargeable lithium batteries depends upon thecharacteristics of electrodes and materials used therein. The energydensity in commercial lithium ion batteries generally decreases as powerdensity increases. For example, U.S. Pat. Nos. 6,337,156 and 6,682,849(incorporated by reference herein in their entirety) describe electrodesfor secondary batteries, though it has been observed that the electrodesas disclosed do not often provide satisfactory high power and highenergy density levels. Moreover, lithium metal phosphate electrodes asdescribed in EP 1722428 (incorporated by reference herein in itsentirety) for secondary batteries in the prior art often display poorrate behavior, and therefore their capacity at high rates, e.g. at 2 C,is often far away from the desired capacity.

Therefore, a need exists for improved high power lithium secondarybatteries with good high rate behavior and methods of manufacturingrelated electrodes therein.

SUMMARY OF THE INVENTION

The invention is related to secondary lithium battery systems andmethods for their manufacture. Various aspects of the inventiondescribed herein may be applied to the applications set forth below orfor any other types of lithium batteries. The invention may be appliedas a standalone system or method, or as part of an integrated battery orelectricity storage system. It shall be understood that differentaspects of the invention can be appreciated individually, collectively,or in combination with each other.

One aspect of the invention provides a secondary battery with anelectrode. The electrode has a conductive substrate coated with aplurality or powder of flakes. The flakes may have a number averagelongest dimension of greater than 60 and the flakes may be made of ametal oxide or metal phosphate material. In some embodiments of theinvention, the plurality of flakes may be aligned to form a cathodecoating that is at least 30 μm thick. In some embodiments of theinvention, the flakes may have a shortest dimension of about 17 μm or 25μm. The flakes may be monocrystalline or polycrystalline. In someembodiments of the invention, there may be a filler of a powder that maybe metal oxide or metal phosphate material or a combination thereof,which may fill in the spaces or voids between the plurality of flakes.

In some embodiments of the invention, the metal oxide material may beselected from the group consisting of LiCoO₂, LiMn₂O₄,Li(M1_(x1)M2_(x2)Co_(1-x1-x2))O₂ where M1 and M2 are selected from amongLi, Ni, Mn, Cr, Ti, Mg, or Al, 0≤x1≤0.5 and 0≤x2≤0.5. In someembodiments of the invention, the metal oxide material may beLiM1_((1-x))Mn_(x)O₂ where 0<x<0.8 and M1 represents one or more metalelements.

Another aspect of the invention provides a method for making a positiveelectrode material for a secondary battery. The method involves a stepof preparing flakes of a cathode active material. Next, flakes of adesired size are separated by passing them through and onto theappropriate metal screens. A flake slurry is then prepared by combiningthe classified polycrystalline flakes with a filler powder, a conductivepowder and a binder with a solvent. The slurry is then coated on aconductive substrate. The coated substrate is heated to evaporate thesolvent and then pressed to a desired final electrode thickness.

Other goals and advantages of the invention will be further appreciatedand understood when considered in conjunction with the followingdescription and accompanying drawings. While the following descriptionmay contain specific details describing particular embodiments of theinvention, this should not be construed as limitations to the scope ofthe invention but rather as an exemplification of preferableembodiments. For each aspect of the invention, many variations arepossible as suggested herein that are known to those of ordinary skillin the art. A variety of changes and modifications can be made withinthe scope of the invention without departing from the spirit thereof.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent, or patent application wasspecifically and individually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention may be further explained byreference to the following detailed description and accompanyingdrawings that sets forth illustrative embodiments.

FIG. 1 illustrates a cathode or positive electrode with a coating formedwith flakes manufactured in accordance with the invention.

FIG. 2 is a scanning electron microscope (SEM) image of apolycrystalline flake formed in accordance with the invention.

FIGS. 3A-B are performance charts for embodiments of the inventionillustrating relationships between capacity and discharge current.

FIGS. 4A-B are performance charts for embodiments of the inventionillustrating relationships between capacity and number of cycles.

FIGS. 5A-B are flowcharts describing methods of forming active materialflakes or positive electrode material for secondary battery electrodesin accordance with another aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

While embodiments of the invention are shown and described herein, itwill be obvious to those skilled in the art that such embodiments areprovided by way of example only. Numerous variations, changes, andsubstitutions will be apparent to those skilled in the art withoutdeparting from the scope of the invention. It shall be understood thatsuch alternatives to embodiments of the invention described herein areconsidered as part of the invention.

For the purpose of this description of the invention, the term“particle” shall be construed as any finely dispersed regularly orirregularly formed single structure which may be present in ordered ordisordered crystalline, i.e., in monocrystalline or polycrystalline, oramorphous form. A plurality of primary particles may aggregate to formsecondary or flake structures in accordance with the invention.Alternatively, secondary structures or particles may agglomerate to formtertiary or flake structures. The term “flake” may be construed as aplurality of individual particles or secondary (intermediary) structuresthat in turn are composed of primary particles.

FIG. 1 illustrates an electrode manufactured in accordance withprinciples of the invention. The electrode may be constructed as acathode component for lithium secondary batteries. A cathode or activematerial coating having a predetermined thickness 140 may be applied ordeposited onto a substrate layer 100. The coating may include aplurality of elongated structures or flakes 120 formed in accordancewith another aspect of the invention. A filler 130 may be also includedas part of the cathode coating in combination with the flakes 120.Moreover, the coating may be disposed in fluid contact with anelectrolyte 150. The interaction between an active material and anelectrolyte in a battery is well known. Any electrolyte appropriate foruse in a lithium battery can also be used with the positive electrodesprovided herein. For lithium ion battery applications, lithium ionsduring a discharge phase will move rapidly through the electrolyte 150to become intercalated into the active material.

A variety of electrolytes may be selected for use with the inventionincluding those in any form, such as liquid, semi-solid, or even solid.The electrolyte should cooperate with active electrode materials toprovide chemical reactions which store and release electrical energy,and many such chemistries are already known. For lithium ion batteryapplications, an electrolyte can be generally selected from lithium ionconducting chemicals such as lithium hexafluorophosphate in ethylenecarbonate and dimethyl carbonate. Also, for safe operation of the cells,the electrolyte may be preferably selected from a non-flammable group ofchemicals.

It shall be understood that while FIG. 1 depicts a single layer cathodecoating, the number of layers may vary in accordance with the inventionfrom electrode to electrode, and battery to battery depending onselected applications. The coating for example is commonly double sidedor layered so that both sides of the substrate have a layer of coating.

The coating may comprise elongated structures or flakes with relativelygreater average longest dimensions. The elongated structures or flakesprovided herein tend to lie down flat on and generally parallel to asubstrate, and are therefore less likely as such to permeate or pierceadjoining separator sheets or films. Furthermore, these longer flakesmay form relatively thinner cathode coatings which are distinguishablefrom other known particles that are relatively shorter yet formrelatively thicker cathode coatings. The flakes provided herein haverelatively greater average longest dimensions and thus tend to settleand lie flatter than shorter particles, which can even lie uprightrelative to a substrate within a coating but not penetrate the coatinglayer. For example, a relatively thin cathode coating may be formed thatis preferably at least 30 μm thick with a plurality of substantiallyaligned flakes therein. In other embodiments of the invention, a cathodecoating may be provided with a thickness of about 35, 40, 45, 50, 55,60, 65, 70, 75, 80, 85, 90, 95 or 100 μm. Moreover, the inter-particleconnectivity provided within the electrode coatings herein mayfacilitate electron transport. The coatings include active materialflakes that can be densely packed with significant inter-particleconnectivity. The flakes can thus experience a physical joining withoutan interface or boundary. The lack of inter-particle connectivity mayotherwise require electrons to hop across or tunnel through to anadjacent or next particle. The coatings provided herein thus containactive material flakes having a unique morphology that offers relativelyhigh inter-particle connectivity. At the same time, the flakes mayachieve general physical alignment within the coatings whereby flakeswithin any given layer has multiple points of contacts with those fromadjacent layers. This property may at least in part provide secondarybatteries with increased power and energy density compared to othermorphologically different materials and particle shapes.

It shall be understood that the electrode shown in FIG. 1 may be asection of a rechargeable secondary battery, and while not shown,another electrode (anode) can be assembled and combined as known tothose of ordinary skill in the field.

Flake Dimensions

The flakes provided in accordance with the invention can be manufacturedfor various secondary batteries. These elongated structures may belayered as part of a coating onto a conductive substrate to form anelectrode. The starting active materials selected herein may providevarious flake or particle sizes allowing for its use as a positiveelectrode material in accordance with the invention.

In general, the secondary particle or flake sizes herein range from anumber average longest dimension from about 60 to 200 μm. A plurality offlakes may be stacked and interwoven within a cathode coating, whereinthe flakes preferably have a number average longest dimension preferablygreater than about 60 μm. The flakes may be also formed with a numberaverage longest dimension preferably greater than 65, 70, 75, 80, 85,90, 95, 100, 110, 120, 130, 140, 175, 185, 190 or 200 μm.

Larger flake sizes greater than 50 μm of cathode active material aregenerally preferred. Such elongated structures or flakes are noticeablylonger or have significantly greater longest dimensions than knownactive material structures or particle morphology. In addition, theflakes may be formed with a smallest or shortest dimension of about 17,19, 21, 23 or 25 μm. Meanwhile, the flakes or secondary particlesprovided herein may be in turn formed from individual primary particles.The primary particles may have a size ranging from about 10-500nanometers (10⁻⁹ m) (longest dimension). The use of primary particles inthese (nano) ranges may thus improve the packing density andinter-particle connectivity of selected active materials. Any activematerial suitable for use as a cathode material may be selected asbuilding block or primary particle to form higher order secondaryparticles or flakes in accordance with the invention to form a positiveelectrode for a lithium battery.

The use of metal oxide flakes as cathode active materials providedherein has been observed to give rise to high power and high energybatteries. Relatively smaller flake structures have been previouslydescribed for use with secondary batteries utilizing a lithiated oxidecathode and either a titanium disulfide or a carbon anode. (See, U.S.Pat. Nos. 6,337,156 and 6,682,849—Narang). Such coatings howeverincluded fine particles measuring 20 to 50 μm in the longest dimension,which are generally of the shape of prolate spheroids with aspect ratiosin orthogonal x, y and z axes averaging approximately 3:1:1. Meanwhile,the elongated structures which are larger than the above particlesprovided in accordance with the invention with preferable aspect ratiosof approximately 6:6:1 surprisingly demonstrate improved performancecharacteristics. These unexpected results may be attributed at least inpart to the flake morphology and relatively longer structuraldimensions, whereby the longest dimension of these structures fallwithin a range of substantially larger than 50 and preferably greaterthan 100 During experimentation, the flake or tertiary particle sizeswere measured by passing the active material through screens which haveapproximately square openings. The screen sizes used (in U.S. Meshsizes) can be 100, 150 and 230, which correspond respectively to 150,105 and 63 μm. Observation of SEM images (see FIG. 2) may confirm thesize of the flakes and tertiary structures provided herein. The largestdimension of formed flakes may therefore be at least the dimension of aselected screen size, and can be significantly longer, resulting inflakes with longest dimensions larger than 63, 105 and 150 μm.

Active Materials

The flakes and elongated structures provided in accordance with theinvention may be formed from a variety of active materials including oneor more metal oxides or metal phosphates.

Examples of active materials that can be used for constructing theflakes herein include metal oxides such as lithium cobalt oxide(LiCoO₂). For example, concepts of the invention may be applied to theLiCoO₂ compositions described in U.S. Pat. Nos. 6,337,156 and 6,682,849(Narang) to construct cathodes having elongated active material flakesfor use in high power batteries.

Other active materials that may be selected herein include LiMn₂O₄,Li(M1M2Co)O₂ where M1 and M2 are selected from among Li, Ni, Mn, Cr, Ti,Mg, or Al. Alternatively, the invention may incorporate a metal oxidematerial that is a composite with Li₂Mn₂O₃, where the other component ofthe composite has a layered or spinel type structure as described inU.S. Pat. No. 7,303,840 (Thackeray), which is incorporated by referenceherein in its entirety. In some embodiments of the invention, the metaloxide material may be selected from the group consisting of LiCoO₂,LiMn₂O₄, Li(M1_(x1)M2_(x2)Co_(1-x1-x2))O₂ where M1 and M2 are selectedfrom among Li, Ni, Mn, Cr, Ti, Mg, or Al, 0≤x1≤0.5 and 0≤x2≤0.5. In someembodiments of the invention, the metal oxide material may beLiM1_((1-x))Mn_(x)O₂ where 0<x<0.8 and M1 represents one or more metalelements.

Alternatively, metal phosphate materials may be also selected having theformula LiMPO₄, wherein M is selected from one or a combination of Fe,Mn, Ni or Co. For example, an embodiment of the invention may provideelectrochemical cells in which a cathode is comprised of such metalphosphate compound as described in U.S. Pat. No. 5,910,382 (Goodenough).

In preferable embodiments of the invention, the metal oxide material mayinclude cathode compositions and nickel-manganese-cobalt (NMC) materials(3M Innovative Properties Company, Battery Cathode Materials, BC-618,BC-718 and BC-723) including those described in U.S. Pat. No. 6,964,828which is incorporated by reference herein in its entirety. Electrodesutilizing such materials can be described as LiM1_((1-x))Mn_(x)O₂ where0<x<0.5, wherein M1 represents one or more metal elements, and in otherembodiments, M1 includes nickel, cobalt or a combination thereof. A morepreferable active material for selected cathodes herein includes thespecific material LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂ (NMC). It has beenobserved that such electrodes comprising flakes made from NMC inaccordance with the invention perform better at high rates of dischargein both pulse mode and continuous mode than electrodes made fromparticles of NMC.

The initial NMC materials used in accordance with the invention can madefrom a very fine powder consisting of primary particles. These primaryparticles may be generally in the shape of spheres with a diameter ofapproximately 8 μm or less, and preferably 6 μm for certainapplications. Alternatively, the NMC powder may consist of secondaryparticles that in turn are made from primary particles of nanometerdimensions. The NMC can be used to construct secondary structures orflakes provided herein, which may preferably have a longest dimension of50 μm, 105 μm, or 150 μm or less as measured by a selected screen sizeused to sieve the flakes. In preferable embodiments, the flakes are alsopolycrystalline materials made of very fine primary crystals. Aspreviously mentioned, it has been observed that positive electrodescomprising NMC flakes or elongated structures herein demonstratedexcellent performance at high rates as compared to the electrodes usingonly the powder form of NMC. Accordingly, preferable embodiments of theinvention may provide NMC batteries for various battery sizes and types,including commonly used 18650 cells, for power tools, mobile andportable electronic devices.

Fillers

A variety of fillers may be incorporated with active materials as partthe positive electrode (cathode) coatings herein. One or more fillersmay be selected for filling in spaces or voids between the flakes orelongated structures within the coating. A preferable filler maycomprise a powder with average particle size less than about 17 μm inany direction. Other alternative fillers may consist of a metal oxide ora metal phosphate material or a combination of metal oxide and metalphosphate materials.

Current Collectors

The active material flakes or elongated structures provided herein canbe incorporated into coatings that are deposited onto a currentcollector or conductive substrate according to known techniques.Exemplary materials for current collectors or conductive substratesinclude aluminum, copper, nickel, steel and titanium. Current collectorsor substrates herein may be configured into various forms includingcylindrical structures, grids and foils. Any current collectorappropriate for use in a lithium battery can be selected. It shall beunderstood that the active materials provided in accordance with theinvention can be incorporated into a variety of lithium battery formatsand configurations, including but not limited to 18650 cylindrical celltype lithium ion batteries.

FIG. 2 is a scanning electron microscope (SEM) image of flakes providedin accordance with the invention. The particular flake sizes and shapesherein may be formed with different active materials under variousconditions, and may thus vary in morphology. For example, a preferableembodiment of the invention incorporates an NMC powder with smallprimary particles as described elsewhere herein. The small particles maybe combined in accordance with another embodiment of the invention toform secondary structures as can be seen in the SEM. One or moresecondary structures can be combined in turn to form the overall flakestructures provided herein. The secondary structures may have apreferable average particle size of 6 μm. Therefore, preferableembodiments of the invention may be described as having three ranges ortypes of particles: primary particles in the nanometer range, secondaryparticles in the 6 μm range, and flakes with an average number longestdimension in a preferable range of 50 μm and greater. It has beenobserved that improved battery performance can be achieved withdifferent size flakes, including some in the 63 μm range and evenlarger.

The flakes or secondary particles as shown in FIG. 2 may bemonocrystalline, or preferably polycrystalline. These structures can bemanufactured in accordance with another aspect of the invention asdescribed elsewhere herein.

FIGS. 3A-B and 4A-B are graphs that describe the performance forexemplary flakes formed in accordance with the invention as may bedepicted in FIG. 2. It has been observed that with some flakes similarbattery performance may be achieved independent or regardless of actualflake size.

For example, cathode coatings with two different flake sizes weretested: a first flake size of 17 μm (shortest dimension)×150 μm (longestdimension), and a second flake size of 17 μm×63 μm. A pulse test wasconducted using each flake size with results shown in FIG. 3A, wherein35 A pulses were delivered for 10 second periods (pulse length).Comparable battery performance was achieved as illustrated with bothflake sizes with respect to the number of pulses and measured watt-hrs(Wh). In addition, a rate test was conducted using each flake size withresults shown in FIG. 3B, wherein eight different discharge currentlevels were tested and after charging at 1.3 A. Discharge was at C/5, C,5 A, 10 A, 15 A, 20 A, 25 A and 30 A, while charging at 1.3 A after eachdischarge current. Similar performance was observed for the batterieswith different flake sizes at each discharge current level.

Other testing was carried out with cathodes incorporating the two flakesizes as shown in FIGS. 4A-B. More than 200 cycles were achieved atrelatively high rates regardless of flake size. As shown in FIG. 4A, theamount that the battery capacity diminished over the number of cycleswas comparable for each flake size. During this cycle test, dischargewas tested at 5 A, and charged at 1.3 A (4.2-2.5V; RT). Another cycletest with higher energy pulses was also conducted and yielded similarrelative performance data. As shown in FIG. 4B, capacity diminished at agreater rate over the number of cycles as expected but the rate wascomparably the same for each flake size. About 208-212 cycles wereachieved at 10 A discharge rate before capacity retention dropped belowa desired 80% level. During this cycle test, discharge was tested at 10A, and charged at 1.3 A (4.2-2.5V; RT).

FIGS. 5A-B are flowcharts describing methods of forming active materialflakes and positive electrode for secondary batteries. For example, apreferable embodiment of the invention provides a method for initiallymaking or forming active material flakes that can be subsequently usedfor positive electrode coatings. A cathode active material such as thosedescribed elsewhere herein may be selected such as NMC powder. Thepowder may consist of spherical primary particles with average particlesizes ranging from 10 to 0.50 μm, or more preferably from 6 to 1 μm. Aslurry may be prepared by adding a binder and a solvent as known tothose of skill in the field. In the formation of active material flakesprovided herein, any appropriate binder and solvent may be used. Anexemplary binder includes preferably polyvinylpyrrolidone (PVP), and anexemplary solvent includes preferably isopropyl alcohol (IPA). Theslurry may be applied on a substrate and dried in accordance withdesired parameters. An exemplary substrate includes polyethylene (PE)though other polymer films and materials can be used as known to thoseof ordinary skill in the field. Following drying using known equipmentsuch as convection dryers, shredders or sieves, the active material maybe fragmented or broken up into flakes or elongated structures.

The following two additional steps are further performed in accordancewith the invention: sintering the flakes at a desired temperature orrange of temperatures for various periods of time, preferably at 400° C.to 1100° C. for 1 to 48 hours; and separating the flakes to isolatethose with a desired size by passing the sintered structures onto andthrough the appropriate metal screens. It shall be understood that anyof one or more steps shown in FIG. 5 may be optionally carried out andmay be performed in different sequences according to selectedapplications.

Sintering

In preferable embodiments of the invention, the active material flakesor tertiary particles for positive electrodes are subjected to asintering process. The flakes, which can be formed from agglomerates ofsmaller primary particles, are often characterized as being in a “green”state prior to sintering (aka “green flakes”). Subsequently, the flakescan be sintered in a heating apparatus such as an oven or furnace so asto bring about the physical joining of the primary particles and provideinter-particle connectivity. For example, primary particles of NMCactive material can be sintered under various conditions which result inthe physical joining of active material particles thus forming higherorder flakes and/or tertiary particles. It has been observed generallythat longer sintering times are called for at a lower temperature, andvice versa.

The flakes may be sintered at one or more desired temperatures or rangesover one or more selected periods of time. For example, flakes may besintered according a combination of any of the following temperaturesand/or time periods: a temperature of approximately 400, 500, 600, 700,800, 900, 1000, 1100 or 1200° C.; a time period of approximately 1, 5,10, 15, 20, 25, 30, 35, 40, 45, 46, 47, 48, 49 or 50 hours. The flakesprovided in accordance with the invention can be sintered under anyconditions that may result in the physical joining of active materialparticles so as to provide desired inter-particle connectivity.

Classification/Categorization of Flakes

The flakes formed in accordance with this aspect of the invention mayvary in size depending on various conditions. As known to those of skillin the field, these flakes may be observed through SEM photographs tostudy and determine the actual flake sizes on a mass (or number) averagebasis. It is preferable to classify or categorize the flakes orelongated structures herein according to their sizes with conventionalseparation systems and methodologies.

For example, sieving and screening are methods of separating a mixtureor grains or particles into 2 or more size fractions. The oversizedparticles of materials are trapped above a screen, while undersizedmaterials can pass through the screen. Sieves can be used in stacks, todivide samples up into various size fractions, and hence determineparticle size distributions. Sieves and screens are usually used forlarger particle sized materials, i.e., greater than approximately 50).tm(0.050 mm).

Two scales commonly used to classify particle sizes are the US SieveSeries and Tyler Equivalent, sometimes referred to as Tyler Mesh Size orTyler Standard Sieve Series. The most common mesh opening sizes forthese scales are given in the table below and provide an indication ofparticle sizes.

US Sieve Tyler Opening Size Equivalent mm in — 2½ Mesh 8.00 0.312 — 3Mesh 6.73 0.265 No. 3½ 3½ Mesh 5.66 0.233 No. 4 4 Mesh 4.76 0.187 No. 55 Mesh 4.00 0.157 No. 6 6 Mesh 3.36 0.132 No. 7 7 Mesh 2.83 0.111 No. 88 Mesh 2.38 0.0937 No. 10 9 Mesh 2.00 0.0787 No. 12 10 Mesh 1.68 0.0661No. 14 12 Mesh 1.41 0.0555 No. 16 14 Mesh 1.19 0.0469 No. 18 16 Mesh1.00 0.0394 No. 20 20 Mesh 0.841 0.0331 No. 25 24 Mesh 0.707 0.0278 No.30 28 Mesh 0.595 0.0234 No. 35 32 Mesh 0.500 0.0197 No. 40 35 Mesh 0.4200.0165 No. 45 42 Mesh 0.354 0.0139 No. 50 48 Mesh 0.297 0.0117 No. 60 60Mesh 0.250 0.0098 No. 70 65 Mesh 0.210 0.0083 No. 80 80 Mesh 0.1770.0070 No. 100 100 Mesh 0.149 0.0059 No. 120 115 Mesh 0.125 0.0049 No.140 150 Mesh 0.105 0.0041 No. 170 170 Mesh 0.088 0.0035 No. 200 200 Mesh0.074 0.0029 No. 230 250 Mesh 0.063 0.0025 No. 270 270 Mesh 0.053 0.0021No. 325 325 Mesh 0.044 0.0017 No. 400 400 Mesh 0.037 0.0015 Source:www.AZoM.com

The mesh number system is a measure of how many openings there are perlinear inch in a screen. Sizes vary by a factor of √2. This can easilybe determined as screens are made from wires of standard diameters,however, opening sizes can vary slightly due to wear and distortion. USSieve Sizes differ from Tyler Screen sizes in that they are independentscales based on arbitrary numbers.

Preparing Flake Slurry

A predetermined quantity of classified flakes may be selected, withflakes that are formed in accordance with other aspects of theinvention. A slurry of the active material flakes can be prepared byadding a selected filler powder, a conductive powder, a binder and asolvent. In the formation of active material flake slurries providedherein, any appropriate binder and solvent may be used. Exemplaryconductive powders or agents include carbon black, acetylene black,KETJEN BLACK, Super-P, PureBlack, natural graphite, synthetic graphite,or expanded graphite. In some embodiments, the conductive agents may bea blend of the above. The added carbon herein is not limited to specificgrades, carbon sources or manufactures thereof. Exemplary bindersinclude preferably polyvinylidene fluoride (PVDF),polytetrafluoroethylene (PTFE), ethylene propylene dieneter-polymer/monomer/M class rubber (EPDM) and polyvinyl alcohol (PVA).Exemplary solvents include preferably N-methyl-2-pyrrolidone (NMP), analcohol (ethanol) and an alcohol/water mixtures.

In alternative embodiments of the invention, the flakes may be formedwith a carbon coating which is in intimate contact with their surface.This may increase the capacity and overall conductivity of an electrode.

It has been observed that the flakes or elongated particles herein canbe easily handled when included in a flake slurry. While theserelatively larger structures are formed with relatively greater longestdimensions, they were found to provide relatively smooth electrodecoatings after solvent evaporation, and especially after compressing theelectrodes. This suggests that applying a flow field through knowntechniques such as through a roll coating process may encourage theflakes to lie flat in a coating. A final compression may be applied by auniaxial press or calendaring machine. The flake slurries may providecathodes with very high rate capability as demonstrated by the testingof cells containing them described elsewhere herein.

Coated Substrate

The positive electrode materials comprising the flakes herein may bedeposited on a variety of substrates. For example, a conductivesubstrate such as metal foils may be used as known to those of skill inthe field. Exemplary materials for substrates include aluminum, copper,nickel, steel and titanium. Accordingly, said flakes may be alignedwithin a deposited cathode coating, whereby the thickness of the coatingon the substrate is preferably less than 50, or preferably greater 30μm, for certain embodiments of invention.

Many known methods can be used for coating a conductive substrate withactive electrode materials described herein. Typical methods includespray coating or spray deposition and techniques such as those describedin U.S. Pat. No. 5,721,067 (Jacobs et al.), U.S. Pat. No. 4,649,061(Rangachar) and U.S. Pat. No. 5,589,300 (Fauteux). Alternatively, othermethods to form a coating include roll coating, casting,electrospraying, thermal spraying, air spraying, ultrasonic spraying,vapor deposition, powder coating and other known techniques.

Compression of Coated Substrate

An advantage of using cathode materials formed with flakes or elongatedstructures as provided herein is that they can easily be aligned withadditional pressure upon manufacture of the positive electrode. Applyinga controlled amount of compressive or uniaxial force to the flakes canmanipulate the flakes so as to rearrange them in a favorable manner andorientation. For example, their relatively flatter faces can be alignedsubstantially perpendicular to the direction of an applied force ontothe coating. Other advantages of the flake shape are that it provides alarge surface area per weight or per volume and a higher packing densityas compared to other geometries, thus providing relatively higherelectrode energy density for electrodes provided herein.

The cathode coatings provided herein may form upon application of acompressive force or pressure. At the same time, the flakes or elongatedstructures therein may preferably form closely packed regular orirregular interwoven stacks, thus bringing them in close contact to oneanother.

The electrode coatings herein may be deposited and layered ontocollector foils or substrates through various known processes such asthrough a roll coating processes.

In alternative embodiments of the invention, a further step ofcompacting of the flakes by equipment may be performed using anapparatus such as a roll mill to improve the packing density of thecoating. These coatings containing dense positive electrode activematerial structures may already have sufficient porosity to be wetted byan electrolyte. But their porosity may be further modified for certainapplications as known to those of ordinary skill in the field.

Other embodiments of the invention may comprise a further or alternativestep of densifying a dried coating by various other means in addition toapplying uniaxial pressure, wherein the densifying step aligns theparticles along or in a preferred direction or orientation. For example,a final densification procedure can be carried out by means of a platenpress or a calender press or any other suitable means. A calendaringstep may be also carried out two times (two-pass calendaring) or more inorder to achieve a desired level of densification. The densificationstep may be preferrably carried out to provide a greater alignmenteffect of flakes within an electrode coating. Plus an increased physicalcontact of the flakes can be achieved as compared to an electrodeobtained in a process without the additional step of densification. Itis preferable that an applied pressure is a uniaxial pressure which mayincrease the electrical and ionic conductivity and capacity of theresulting electrodes.

Preferable embodiments of the invention include a densification stepthat is carried out using a roll mill with a line pressure applied in awide range from 3000 to 9000 N/cm, preferably 5000 to 7000 N/cm forcertain applications. The selected ranges for the line pressure appliedcan provide the desired alignment of elongated structures in a preferreddirection within an electrode coating, thus generating a desiredelectrode structure. As explained in the foregoing, the elongatedstructures are preferably configured in the form of flakes andsubstantially aligned along a common plane.

It shall be understood that other embodiments of the invention do notinvolve the aforementioned densification or compression steps. Forcertain applications, an alignment of the flakes or elongated structurescan be observed regardless, and such alignment may be attributed atleast in part to the inherent densification during the manufacturingprocess, when the flakes are aligned as a coating is deposited onto asubstrate. It may be preferable that the flakes are coated onto asubstrate and compressed in preparation of an electrode electricallyconductive of the single flakes. But already by the application of theactive material composition onto the substrate, an alignment can beachieved. Furthermore, the form of the flakes may depend on theconditions of crystallization which are subject to routineexperimentation of a person skilled in the art. In preferableembodiments, the formed structures are in the form of polycrystallineflakes. The particular size and longest dimensions of these flakes arenot of utmost importance for many applications. Though for the purposesof the invention, it is preferable that the structures are generallyarranged flat and substantially aligned within a coating.

Double Sided Coating

In alternative embodiments of the invention, electrodes may be formedwith double sided coatings. A double sided coating may be constructed byfollowing the coating steps described above twice for a selectedsubstrate. A flake slurry may be coated on a first surface of aconductive foil or substrate, and a subsequent step of inverting thecoated foil may be performed prior to pressing. A substantiallyidentical or other active material coating can be applied to a second(opposite) surface of the conductive foil. Thereafter a compressing stepmay be performed by pressing the double sided coated foil to the desiredfinal electrode thickness.

Preparation of Batteries

The positive electrode material provided herein may be used inmanufacturing rechargeable lithium secondary batteries. A positiveelectrode (cathode) can be manufactured by initially preparing a slurrywith a variety of classified or categorized flakes formed in accordancewith the invention in combination with a selected filler powder, aconductive powder, a binder and a solvent including those describedelsewhere herein. The slurry may be then coated onto a conductivesubstrate, followed by drying or heating the coated substrate toevaporate the solvent, and then pressing the coated substrate to adesired final electrode thickness. The following provides a furtherdescription of these steps in accordance with preferable embodiments.

Examples—Electrochemical Cell Preparation

The invention is further illustrated by way of the following exampleswhich are not meant to limit the scope of the invention. It shall beunderstood that the following steps and materials, including knownalternatives to those of ordinary skill in the field and combinationsthereof, fall within the scope of the invention. Some electrodeembodiments of the invention were prepared as follows:

Cathodes

Exemplary cathodes can be prepared as follows with an initial mixturecontaining (percentage by weight): 85-95% of NMC(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂,) (3M); 1-11% Ace Black carbon black(Soltex); and 1-5% graphite grade ABG1010 (6.0% graphite grade ABG1045,Superior Graphite). The ingredients can be mixed in a 1-liter jar withalumina grinding media (½″ size) for 1.5 hour. Then an additional 4-14%by weight of Kynar grade 761 (Arkema) can be added, and the powdersmixed for another 15 minutes. The above mixture can be referred to asthe “cathode dry mix.” The cathode dry mix can be transferred to aplastic bowl to which 80-130% by weight of NMP (N-methyl pyrrolidone)(Sigma Aldrich) is added. The content of the bowl can be mixed to formthe cathode slurry on a shaker for 30 minutes. Then 18-28% by weight ofNMC flakes can be then added to the slurry. The slurry may be shaken foranother 15 minutes.

The cathode can be made by coating an aluminum foil (20 μm thick, 11inch wide) with the cathode slurry on a reverse roll coater. The loading(weight of coating per unit area) can be about 14 mg/cm² per side. Bothsides of the aluminum foil can be coated. After coating, the roll ofcathode can dry in a vacuum oven at about 120° C. for about 10 hours.

After vacuum drying, the cathode can be calendered between two rolls toabout 120 μm thick. The calendered cathode can be then slitted to 54 mmwide and cut to the desired length (about 72 cm). A strip of aluminum(100 μm thick, 4 mm wide) can be ultrasonically welded to the copperfoil near the end of the foil to form a tab.

Anodes

Exemplary anodes can be prepared as follows with an initial mixturecontaining (percentage by weight): 95-99% carbon, grade CPreme G5(Conoco Phillips) and 1-3% Ace Black carbon black (Soltex). Theingredients can be mixed in a 1-liter jar with alumina grinding media(½″ size) for 1.5 hour. Then an additional 0.1-0.9% by weight of oxalicacid (Sigma Aldrich) and 2-12% by weight of Kynar grade 761 (Arkema) canbe added and the powders mixed for another 15 minutes. The above mixturecan be referred to as the “anode dry mix.” The anode dry mix can betransferred to a plastic bowl to which 240-290% by weight of NMP(N-methyl pyrrolidone) (Sigma Aldrich) can be added. The content of thebowl can be mixed to form the anode slurry. The mixing may be done on ashaker for 30 minutes.

The anode can be made by coating a copper foil (13 μm thick, 11 inchwide) with the anode slurry on a reverse roll coater. The loading(weight of coating per unit area) can be about 7.35 mg/cm² per side.Both sides of the copper foil can be coated. After coating, the roll ofanode can dry in a vacuum oven at about 120° C. for about 10 hours.

After vacuum drying, the anode can be calendered between two rolls toabout 110 μm thick. The calendered anode can be then slitted to 56 mmwide, and cut to the desired length (about 74 cm). A strip of nickel(100 μm thick, 4 mm wide) may be ultrasonically welded to the copperfoil near the end of the foil to form a tab.

It should be understood from the foregoing that, while particularimplementations have been illustrated and described, variousmodifications can be made thereto and are contemplated herein. It isalso not intended that the invention be limited by the specific examplesprovided within the specification. While the invention has beendescribed with reference to the aforementioned specification, thedescriptions and illustrations of the preferable embodiments herein arenot meant to be construed in a limiting sense. Furthermore, it shall beunderstood that all aspects of the invention are not limited to thespecific depictions, configurations or relative proportions set forthherein which depend upon a variety of conditions and variables. Variousmodifications in form and detail of the embodiments of the inventionwill be apparent to a person skilled in the art. It is thereforecontemplated that the invention shall also cover any such modifications,variations and equivalents.

What is claimed is:
 1. A secondary battery having an electrodecomprising a conductive substrate coated with a plurality of flakes thathave a number average longest dimension greater than 60 said flakesfurther comprising individual primary or intermediate secondaryparticles formed of a metal oxide or metal phosphate material.
 2. Thesecondary battery of claim 1, wherein the plurality of flakes arealigned to form a cathode coating that is at least 30 μm thick.
 3. Thesecondary battery of claim 1, wherein the flakes have a shortestdimension of about 17 μm or 25 μm.
 4. The secondary battery of claim 1,further comprising a filler for filling in spaces or voids between theplurality of flakes, said filler comprising a powder with averageparticle size less than about 17 μm in any direction.
 5. The secondarybattery of claim 4, wherein the filler consists of a metal oxide or ametal phosphate material or a combination of metal oxide and metalphosphate materials.
 6. The secondary battery of claim 1, wherein themetal oxide material is selected from the group consisting of LiCoO₂,Li(Mn_(1-x3)M3_(x3))₂O₄, Li(M1_(x1)M2_(x2)Co_(1-x1-x2))O₂ where M1 andM2 are selected from among Li, Ni, Mn, Cr, Ti, Mg, or Al, M3 is selectedfrom one or a combination of Li, Ni, Co, Cr, Ti, Mg, or Al, ≤x1≤0.5,0≤x2≤0.5 and 0≤x3≤0.5.
 7. The secondary battery of claim 1, wherein themetal oxide material is LiM1_((1-x))Mn_(x)O₂ where 0<x<0.8 and M1represents one or more metal elements.
 8. The secondary battery of claim7, wherein M1 includes nickel, cobalt or a combination thereof.
 9. Thesecondary battery of claim 7, wherein the metal oxide isLiNi_(0.33)Co_(0.33)Mn_(0.33)O₂ (NMC).
 10. The secondary battery ofclaim 7, wherein the metal phosphate material is described by theformula LiMPO₄, where M is selected from one or a combination of Fe, Mn,Ni or Co.
 11. A secondary battery having an electrode comprising aconductive substrate coated with a plurality of polycrystalline ormonocrystalline flakes of one or more metal oxide or metal phosphatematerials, wherein the flakes have a longest dimension of greater than60 μm and further comprising individual primary or intermediatesecondary particles formed of a metal oxide or metal phosphate material.12. The secondary battery of claim 11, wherein the flakes have ashortest dimension of about 17 μm or 25 μm.
 13. The secondary battery ofclaim 11, wherein the metal oxide material is selected from the groupconsisting of LiCoO₂, Li(Mn_(1-x3)M3_(x3))₂O₄,Li(M1_(x1)M2_(x2)Co_(1-x1-x2))O₂ where M1 and M2 are selected from amongLi, Ni, Mn, Cr, Ti, Mg, or Al, M3 is selected from one or a combinationof Li, Ni, Co, Cr, Ti, Mg, or Al, ≤x1≤0.5, 0≤x2<0.5 and 0≤x3≤0.5. 14.The secondary battery of claim 11, wherein the metal phosphate materialis defined by the formula LiMPO₄ where M is selected from one of or acombination of Fe, Mn, Ni or Co.
 15. The secondary battery of claim 11,wherein the metal oxide material is LiM1_((1-x))Mn_(x)O₂ where 0<x<0.5and M1 represents one or more metal elements.
 16. A positive electrodematerial comprising a plurality of flakes that have a longest dimensiongreater than 100 μm of a metal oxide or metal phosphate material. 17.The positive electrode material of claim 16, wherein the metal oxidematerial is selected from the group consisting of LiCoO₂,Li(Mn_(1-x3)M3_(x3))₂O₄, Li(M1_(x1)M2_(x2)Co_(1-x1-x2))O₂ where M1 andM2 are selected from among Li, Ni, Mn, Cr, Ti, Mg, or Al, M3 is selectedfrom one or a combination of Li, Ni, Co, Cr, Ti, Mg, or Al, ≤x1≤0.5,0≤x2≤0.5 and 0≤x3≤0.5.
 18. The positive electrode material of claim 16,wherein the metal oxide material is LiM1_((1-x))Mn_(x)O₂ where 0<x<0.5and M1 represents one or more metal elements.
 19. The positive electrodematerial of claim 16, wherein the metal phosphate material is defined bythe formula LiMPO₄, where M is selected from one of or a combination ofFe, Mn, Ni, or Co.